The Attribute Refunds system was an early mechanic, at the time the only way to redistribute your attribute points. Spending one refund allowed you to lower one attribute by one rank; increasing an attribute was still free. One refund was gained for each 250 experience gained, and per mission completed (even after the first time). You could have a maximum of 24 attribute refunds at any time.
Unlike today, you could redistribute your attribute points anywhere, even in explorable areas and missions, if you had refunds. Players took advantage of this in some farming builds, changing their attributes in real-time based on what skill they planned to activate next.
The mechanic was removed in the September 7, 2005 update, and replaced with the current attribute point allocation system.